Stationary gas turbines and methods for operating the gas turbines are known in many different forms from the available prior art. Gas turbines of modern type of construction, which are used for generating electrical energy, usually have an axial-throughflow compressor, one or more combustion chambers and a turbine unit. In operation, a fuel delivered to the combustion chamber is burnt with the aid of the ambient air compressed by the compressor, to form a hot gas which expands in the turbine unit at the rotor of the gas turbine so as to perform work. The rotor then drives a generator which converts the mechanical energy into electrical energy with low loss and feeds it into a power distribution network.
When the gas turbine is started, what is known as the start-up or run-up, its rotor is brought to an ignition rotational speed with the aid of a drive device, after which a pilot fuel stream, by being fed into the combustion chamber, is ignited. The pilot flame subsequently ignites a main fuel stream which is also injected into the combustion chamber or combustion chambers via separate burners and/or fuel nozzles.
According to the book “Stationäre Gasturbinen” [“Stationary Gas Turbines”] by Christoph Lechner and Jörg Seume (publishers) the drive device is decoupled from the rotor at a rotational speed of 50% to 80% of the nominal rotational speed. The rotor is then accelerated solely by the hot gas occurring during combustion. When the nominal rotational speed, mostly 3000 min−1 or 3600 min−1, is reached, the operation to start the gas turbine ends. The generator can subsequently be synchronized with the network frequency of the power distribution network and be locked onto this.
The delivery of pilot fuel and main fuel to the corresponding burners or nozzles takes place via separately operating line systems with valves which are arranged in them and by means of which the volume and the fuel delivered in each case and its pressure can be set.
Both liquid and gaseous fuels are used as fuel. To generate especially efficient and low-emission combustion in the combustion chamber, it is known to assist the combustion of the main fuel mass flow constantly by means of the pilot flame. A fuel gas, for example natural gas, is often used as pilot fuel.
On account of the large quantities of gaseous fuel which are required in order to generate large quantities of electrical energy, the fuel line systems of the gas turbine are often connected to a fuel network, from which the fuel can be extracted permanently in the required quantity over a lengthy period of time. If appropriate, an additional gas compressor is also connected between the fuel network and the fuel line system, in order to raise the supply pressure of the fuel network dependably to a higher degree whereby it can be ensured that the gas turbine operates reliably. The fuel pressure required for feeding into the combustion chamber in this case lies above the pressure ratio afforded by the compressor of the gas turbine. Consequently, the fuel pressure is set such that fuel in the required quantity also flows in actual fact into the combustion chamber.
The supply pressure to be provided by the fuel network or to be delivered by the additional gas compressor can even lie well above the pressure ratio afforded by the compressor, since very large quantities of pilot fuel are required particularly during the acceleration of the rotor to nominal rotational speed and during load shedding. Moreover, pilot burners which generate a premixed flame, what are known as premix pilot burners, have comparatively small gas outlet bores which necessitate a further increase in the gas supply pressure which is in any case already high, in order to achieve the required mass flows of pilot gas. This contradicts the need for the possibility of efficient operation even when the supply pressure in the fuel network is reduced.
Moreover, a method for starting a gas turbine operated with low-calorie gas is known from WO 2006/053866 A1. Synthesis gas is injected as low-calorie gas into the combustion chamber of the gas turbine via a pilot burner and a main burner. So as not to undershoot a minimum pressure loss in spite of a comparatively low fuel mass flow, which would lead to too low an outflow velocity, WO 2006/053866 A1 proposes to admix an inert medium to the synthesis gas. However, this has the disadvantage that a high supply pressure is still necessary overall.